Run DEP-8 tests in Xvfb.

It looks like a new version of Pandas introduced Tcl/Tk tests, so DEP-8 tests fail again on Ubuntu's Jenkins: <https://jenkins.qa.ubuntu.com/job/saucy-adt-pandas/5/ARCH=amd64,label=adt/console>.

This PR makes the tests run in X virtual framebuffer (not really tested on an X-less machine, but this works perfect for other packages).